1. Field of the Invention
The present invention relates to printing intelligence on paper. More particularly, the present invention relates to a ribbon shield apparatus for preventing unwanted contact between the ribbon and the paper (causing ink smudges on the paper) during relative movement of the ribbon and paper.
2. Background Art
Several types of impact printers are known today. Many include a fixed platen and a movable print head carriage including a plurality of spaced print wires which are selectively activated and driven into a ribbon and the paper as the head moves across the paper. Such printers are illustrated by U.S. Pat. Nos. 3,592,311 and 3,897,865.
Printers create a substantial amount of noise during print wire activation and contact of the wires with the ribbon, paper and the platen. This noise increases as printers are driven at ever increasing printing rates, now approaching or exceeding 400 characters per second in some printers. This high speed requires more wire activations and platen contacts per unit of time. Current printers generate more noise as greater forces are employed to complete a firing sequence in a shorter period of time (when the print head is in the proper printing location.)
Another problem with printing at ever increasing speeds is that the ribbon is positioned closer to the paper, making it more likely to come into unwanted contact with the paper, leaving smudges and other undesirable markings. In some printers, the print head and the paper are placed in close proximity (perhaps 0.4 millimeter separation) to permit short cycles required in high speed printing, but the ribbon is located between the print head and the paper. Further, the movement of the print head at increased speeds tends to make the ribbon more likely to come into contact with the paper.
One solution which addresses the noise problem as well as the ribbon contact with paper is to mount a ribbon shield on the print head carriage and place it in frictional engagement with the paper. An example of this solution is shown in U.S. Pat. No. 4,410,291. While this reduces the noise created during printing and reduces the unwanted ribbon marking on the paper, the frictional engagement creates wear on the ribbon shield. The wear on the ribbon shield creates failure of the ribbon shield, necessitating periodic replacement. This replacement requirement dictates that an additional supply item (the ribbom shield) be manufactured and distributed so that the user can have his printer repaired as required. This also dictates that a mounting surface and removable mounting hardware be provided in the region of the print head and the ribbon, adding to the already crowded viscinity and possibly requiring greater distance between the print head and the paper. This patent teaches the use of a ribbon shield of approximately 1 millimeter in thickness, which is more than twice as large as the head-to-paper separation in current printers. The ribbon shield of this patent thus could not be used in current printers.
This U.S. Pat. No. 4,410,291 also shares a disadvantage with other printers in that the ribbon shield and the ribbon may be movable with respect to each other. Relative movement of the ribbon with respect to the ribbon shield means that varying portions of the ribbon may be aligned with the print head and an aperture in the ribbon guide at varying times. In the case of a multi-color ribbon, this varying of alignment may mean that a wrong color is printed or that printing partially in two different colors may occur. Both results are, of course, undesirable.
Documents on which printing is desired frequently do not have the stiffness and flatness necessary to feed the paper in the desired path. This is particularly true for a car rental agreement where the calculation of a customer's bill is desired on the form which the customer has been carrying. This carried form may have been folded and its corners bent. In many printers, such a form could either not be handled well or would require manual preparation for printing.
A moving print head carriage has a tendency to snag on portions of paper inserted in a printer. This snagging potential is especially prevalent at edges of paper such as multiple part forms and at "outfolds" where continuous form paper joins together at joints which project out from the platen and into the moving print head.
Cost of manufacturing, assembly and maintenance are considerations regarding all printers and printer ribbons. Many ribbon shield "solutions" proposed previously are expensive either to fabricate and assemble or to maintain. Accordingly, the resulting printer product is either too expensive or relatively uneconomical.
Some printer ribbon assemblies include a small, rather delicate ribbon guide in the vicinity of the print head to achieve a low mass ribbon guide assembly. However, the delicateness of the ribbon guide also allows it to bend stretch and deform during ribbon installation. Any associated structure mounted on the ribbon guide must either have similar capabilities or will break.
Various examples of printer shields are known in the prior art. Examples of these include those shown in U.S. Pat. Nos. 4,165,188; 4,285,604; 4,437,401; and 4,496,256 and in IBM Technical Disclosure Bulletin Vol. 16, No. 3, p. 834; Vol. 26, No. 1, p. 65; and Vol. 26, No. 1, p. 73.
Other limitations and disadvantages of the prior art systems will be apparent to those skilled in the art in view of the description of the present invention.